AutoWeek.com

A trove of one of a kind Concept cars make Joe Brotz's Irreplaceable collection

Heres how it works: DEF is made by combining urea and deionized water. Urea is an organic compound made up of nitrogen, hydrogen, carbon and oxygen. When DEF is mixed with hot exhaust gases, it breaks down into ammonia and carbon dioxide. Once those two gases meet the diesel fumes, they bond with the nitrogen oxides to create nitrogen and water before exiting the tailpipe. In other words, chemistry happens and diesel exhaust is cleaner.